A gyratory compactor is used for defining properties of compaction of various masses of soil and similar substances, such as bituminous asphalt. For measurements the specimen mold of a gyratory compactor is filled with mass specimen. The specimen is compressed under vertical pressure and the so called gyratory movement, which causes shear strain across the specimen. These actions make the particles of the specimen to move in respect with each other finding their positions tighter to each other, compacting the mass specimen and leading to increasing density. At the same time the gyratory compactor frame is experiencing forces due to deformation resistance of the specimen. These forces are greater where the pressure and cross-sectional deformation due to it are greatest. In a gyratory compactor this point is on the direction where the angle of gyration of gyratory motion is turning. Due to this force, the gyratory compactor frame bends (like a stiff spring) usually such that the angle of gyration (gyratory angle) tends to decrease. As the compaction test continues the density and usually also the shear resistance increase. Due to this force, the elastic deformation of the compactor and the decrease of the internal angle of gyration are at minimum in the beginning of the test and at maximum usually at the end of the test.
A change in the angle of gyration is usually the most significant factor of error having influence on results in measuring compaction properties in gyratory compactor. In some present gyratory compactors the angle of gyration in the end of a test may be more than 10% smaller than in the beginning of the test. The compaction results of tests carried out with these kinds of gyratory compactors are examined, however, assuming that the angle of gyration has remained equal during the whole length of the test. Therefore, many different models of gyratory compactors at present give deviating results to each other in measuring compaction properties. The situation is especially serious when the mass specimen is causing heavier loadings or variable loadings to the gyratory compactor (i.e. a “stiff” or coarse mixture).
According to present standards, correlation between the angle of gyration of gyratory compactors is defined by measuring the compacted density of hot mix mass specimens from the same source with different gyratory compactors. This aims to clarify differences between gyratory compactors by comparing test results of different gyratory compactors. Sometimes based on these differences the elastic properties of different gyratory compactors as well as other quality factors are tried to be determined. This kind of studying of tests and their results is rather troublesome and slow. Furthermore, measuring results gathered with this kind of method are inaccurate, among other things, because properties and density of mass specimen vary uncontrollably and relatively much, although the mass specimen has been taken from the same mass specimen source.
An angle measurement device which can be put into a specimen mold of a gyratory compactor is known from the patent U.S. Pat. No. 6,477,783. The device is placed inside a cylinder mold in the bottom or top against the bottom or top piston along with the asphalt mixture. There is a fixed and a movable probe tip against the mold wall, which are placed vertically in a distance from one another. The measuring result from the movable probe tip in relation to the position of the fixed probe tip is in proportion to the angle of gyration of the gyratory compactor. To measure the effective angle of gyration of a gyratory compactor, the device and a hot mix mass specimen is placed in the specimen mold with the hot mix placed above or under the measuring device. The gyratory compactor is then operated in a way corresponding to the normal compacting of mass specimen. From the position between the movable probe tip and the fixed probe tip the angle of gyration of the gyratory compactor is defined during the compaction of mass specimen in the specimen cylinder mold.
There are many disadvantages and imperfections connected to the measuring device of U.S. Pat. No. 6,477,783. Firstly, the measuring device takes part of the space meant for a mass specimen, so that a mass specimen smaller than normal must be used or the compaction pistons are further away from each other than during normal compaction. Therefore angle measurements taken do not correspond to the actual compacting situation of a mass specimen. When the measuring device is shorter in order to decrease the above mentioned disadvantages, the probe tips must be adjusted closer to each other on the vertical axis, which creates more inaccuracy in measuring. Furthermore, the magnitude of loading from a mass specimen to the gyratory compactor is unknown beforehand, and the mass specimen to be placed above or under the measuring device is different by density properties every time when measured, because every time the measurement is made there must be a new mass specimen in the specimen cylinder mold. Therefore, while using the device presented in the patent above several rather long individual test measurements must be carried out if elastic deformations of a gyratory compactor is wanted to define reliably and exactly enough. Furthermore, due to its construction the measuring device is exposed to heat damage while using hot asphalt specimens. Also impurities may stick to probe tips affecting the measuring accuracy. These all are additional to elements of uncertainty in use of the measuring device as well as difficulties in measuring process presented in patent above.